1. Field of the Invention
The present invention relates to a process in which a liquid and a gas are reacted continuously and/or semicontinuously over a fixed-bed catalyst in a reactor through which the reactants are passed in cocurrent and which is equipped with a specific mixing and distribution device for the starting materials.
2. Discussion of the Background
Reactions in which three phases, viz. gas, liquid and solid, participate occur frequently in industrial chemistry. In most cases, the catalyst is present as a solid and the reactants are distributed over the gas and liquid phases. If the reaction occurs on the catalytic surface of the solid, the reaction rate is influenced both by mass transfer between gas and liquid and that between liquid and solid. The mass transfers and the chemical reactions are to be regarded as steps occurring in succession. Each of these steps can be the rate-determining step.
Complex reaction networks frequently occur in chemical processes. Both parallel and subsequent reactions can then proceed simultaneously. In such processes, the selectivity to the target product plays a particularly important role. The conversion and selectivity to the target product depend not only on the reaction kinetics (dependent on, inter alia, the temperature and the pressure) but also on the hydrodynamics of the chosen reactor.
The three-phase reactors used industrially differ essentially in the way in which the phases move. Thus, a distinction is made between suspension reactors (stirred vessel or bubble column), fluidized beds and fixed beds (liquid-filled reactor or trickle-bed reactor).
In a fixed-bed reactor, the catalyst is present in the form of a bed of packing. With regard to the mode of operation, a distinction is made here between a liquid-filled reactor and a trickle-bed reactor. In the trickle-bed reactor, the flow velocity of the liquid phase is low. The reaction gas is conveyed through the reactor from the bottom upward in cocurrent or countercurrent to the liquid phase (Baerns, Hofmann, Renken “Chemische Reaktionstechnik”, Georg Thieme Verlag Stuttgart, 1999, pp. 264-267).
In a three-phase reactor (known as a three-phase trickle-bed reactor), the liquid phase is conveyed from the top downward. The gas phase can flow in the same direction as the liquid phase or in the opposite direction. It is usual to convey both phases (gas and liquid) from the top downward in cocurrent. Three-phase reactors can be operated in various modes, as described below. Depending on the liquid and gas velocities selected, different types of operation are established. At low liquid velocities, the liquid trickles downward in thin films and the likewise relatively low downward-directed gas flow is continuous, resulting in “trickle flow”. When the gas velocity increases and the liquid velocity remains at the same relatively low value, the reactor operates in the “spray flow” region. In contrast, if the gas velocity remains relatively low at a significantly increased liquid velocity, “bubble flow” occurs. If the velocities of the two phases are increased simultaneously, “pulse flow” is obtained. These modes of operation have very characteristic and very different hydrodynamic parameters which have, in particular, an influence on the mass transfer. The conversion and the selectivities of reactions which can be carried out in three-phase reactors depend on the kinetics, the pressure, the temperature and the hydrodynamics of the reactor (Ullmann's Encyclopedia of Industrial Chemistry, Vol. B4, pp. 309-320).
In industrial reactors whose diameter is correspondingly large, the distribution of the starting materials (liquid (starting material 1), gas (starting material 2)) over the total cross section of the catalyst bed plays a critical role. Reactions in which the gas is soluble in the liquid to only a limited extent and the reaction takes place exclusively between the liquid phase and the solid catalyst present an additional challenge. In these cases, the starting material 2 has to be transported from the gas phase into the liquid simultaneously with the progress of the reaction (in the liquid phase). This requires both liquid and gas to be present in sufficient amounts at all places in the reactor, i.e. both reactants have to be distributed optimally both in the radial direction and in the axial direction. In such cases, attempts are made to achieve uniform flow velocities over the entire cross-sectional area of the reactor.
Furthermore, three-phase reactors are usually operated adiabatically, i.e. the temperature alters as the conversion progresses as a result of the heat of reaction which is liberated or taken up and, due to the absence of external heat exchange, increases or drops correspondingly. To achieve a homogeneous temperature distribution in the reactor, a uniform distribution of the starting materials is likewise desirable.
To achieve very good mixing of the starting materials and at the same time a uniform distribution of these over the catalyst surface, various measures have been employed in industry. An inert layer consisting of packing elements, e.g. Raschig rings or spheres, which is intended to ensure uniform distribution of the liquid and the gas phase over the catalyst cross section can be installed above the catalyst.
Another possibility is to carry out the distribution of the starting materials onto the uppermost layer of the catalyst by means of distributor plates, for example perforated plates. A combination of the two abovementioned engineering measures is likewise possible.
U.S. Pat. Nos. 5,882,610 and 6,093,373 describes a mixing and distribution system which comprises a perforated plate in which each hole is provided with an upward-directed tube having lateral drilled holes and a free space between perforated plate and catalyst bed. The space between catalyst and perforated plate has a height of from 0 to 10 cm. The diameter of the tubes is equal to or smaller than half the distance between perforated plate and catalyst bed. The number of tubes per square meter is from 100 to 700. The gas phase and the liquid phase are mixed in the tubes and in the intermediate space before flowing through the catalyst bed. It is stated that the mixing of gas and liquid and the uniformity of the flow of the starting materials onto the catalyst surface are improved by means of the apparatus claimed. However, examples of an effect in carrying out chemical reactions are not given. The height of the reactors used is in each case 4 m at a diameter of 400 mm.
A high ratio of the length of the reactor or length of the reaction zone to the diameter is typical of three-phase reactors. In general, a ratio of greater than 5, preferably from 5 to 25, is proposed for this type of reactors (Ullmann's Encyclopedia of Industrial Chemistry, Vol. B4, p. 310).